This invention relates generally to imaging systems capable of operation in multiple modalities, and more particularly to an apparatus and method for estimating coincidence events generated by a multi-modality imaging system.
Multi-modality imaging systems are capable of scanning using different modalities, such as, for example, Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), and Computed Tomography (CT). During operation of a PET imaging system for example, a patient is initially injected with a radiopharmaceutical that emits positrons as the radiopharmaceutical decays. The emitted positrons travel a relatively short distance before the positrons encounter an electron, at which point an annihilation occurs whereby the electron and positron are annihilated and converted into two gamma rays each having an energy of 511 keV.
The annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence, gamma photons also identify a line of response, or LOR, along which the annihilation event has occurred.
The number of time coincidences, generally referred to as coincidence events, detected within a field of view (FOV) of the detector is the count rate of the detector. The count rate at each of two oppositely disposed detectors is generally referred to as singles counts, or singles. The coincidence event is identified if the time difference between the arrivals of signals at the oppositely disposed detectors is less than a predetermined time coincidence. The number of coincidence events per second registered is commonly referred to as prompt coincidences or prompts. Prompts may include true coincidences and random coincidences. True coincidences are those physically correlated time coincidences, i.e., two gamma photons emitted in the process of annihilation or photons produced from the two primary gamma photons.
In addition to the true coincidence events described above, at least one other type of coincidence event, referred to herein as randoms is detected by the PET scanner. The randoms typically confound the data collection and image reconstruction process particularly at high count rates and in volumetric acquisitions. The phenomenon known as randoms occurs when photons from two different annihilations are detected by two crystals at essentially the same time. Randoms are due to valid events being detected at the same time even though the gamma photons did not originate from the same annihilation. The valid events may also come from other non-annihilation sources. These events are called randoms because it is random chance that the two arrived at the same time. The probability of such a random event occurring is directly proportional to the event rate in each of the two single detectors compared in the coincidence pair. Randoms are deleterious to the PET acquisition because, even if the expected number of random coincidences in an acquisition may be estimated and compensated for in the data set, counting the random coincidence events adds Poisson noise to the data set, reducing the signal-to-noise ratio of the data, and, ultimately the reconstructed PET image.
One method to estimate the rate of random coincidence acceptance is referred to as the Randoms from Singles method. The Randoms from Singles method measures the detected singles counts for each channel in the detector and uses those measured counts to predict the random coincidence coincidences for each detector pair in the prompt channel. The conventional Randoms from Singles method relies on the assumption that the singles rate is constant in the detectors for the duration of the acquisition. For example, a constant singles rate is achieved if the activity distribution does not move during the course of the acquisition, and if the acquisition duration is short compared to the half-life of the radiopharmaceutical being imaged. For most radiopharmaceuticals having a relatively long half-life compared to the acquisition interval, the Randoms from Singles method is effective in estimating and eliminating the randoms.
However, if a study is performed using an radiopharmaceutical having a relatively short half-life, the Randoms from Singles method is less effective. For example, if the study is performed using 82Rb+, which has a half-life of 1.3 minutes, so that imaging frames of several minutes duration are not short compared to the radiopharmaceutical half-life, then the randoms from singles method may cause quantitative inaccuracies and/or artifacts to occur in the image. In some cases, an assumption of simple exponential decay of the radioisotope in the patient can be used to derive a correction factor for the Randoms from Singles estimate. In other cases, such a model is insufficient, and inaccuracies remain in the Randoms from Singles estimation process. For example, because counts are obviously at a premium in these short scans, these frames may be started while the heart is still taking up activity, so the activity distribution is not stationary even if decay is taken into account. Three-dimensional (3D) image reconstructions may be particularly sensitive to these artifacts, due to the higher fraction of random coincidences in the prompt channel and also the sensitivity of the 3D scatter correction tail fit routine to data which is imprecisely corrected for randoms. However, even in two-dimensional (2D) imaging there is the possibility for quantitative errors to be introduced into the reconstructed images.